The 802.11e specification, approved by the Institute of Electrical and Electronics Engineers (IEEE) in late 2005, defines quality of service (QoS) mechanisms for WTRUs which support bandwidth-sensitive applications such as voice and video. The original IEEE 802.11 media access control (MAC) protocol which defines two different access methods, the distributed coordination function (DCF) and the point coordination function (PCF). The DCF is basically a carrier sense multiple access with collision avoidance mechanism (CSMA/CA). CSMA protocols are well known in the industry, where the most popular is the Ethernet, which is a CSMA/collision detection (CD) protocol. Using the CSMA protocol, an AP or WTRU desiring to transmit senses the medium, if the medium is busy, (i.e., some other WTRU or AP is transmitting), and then the AP or WTRU will defer its transmission to a later time when the medium is sensed as being free. These types of protocols are very effective when the medium is not heavily loaded, since it allows stations to transmit with minimum delay, but there is always a chance of stations transmitting at the same time (collision), caused by the fact that the stations sensed the medium free and decided to transmit at once.
These collision situations must be identified, so the MAC layer can retransmit the packet by itself and not by upper layers, which would cause significant delay. In the Ethernet case, this collision is recognized by the transmitting stations which go to a retransmission phase based on an exponential random backoff algorithm.
While these collision detection mechanisms are a good idea on a wired LAN, they cannot be used on a wireless LAN environment, because implementing a collision detection mechanism would require the implementation of a full duplex radio, capable of transmitting and receiving at once, an approach that would increase the price significantly. Furthermore, on a wireless environment, it cannot be assumed that all stations hear each other, (which is the basic assumption of the collision detection scheme), and the fact that a station willing to transmit and senses the medium free, does not necessarily mean that the medium is free around the receiver area.
In order to overcome these problems, the IEEE 802.11 uses a collision avoidance mechanism together with a positive acknowledge scheme. A station willing to transmit senses the medium. If the medium is busy, it then defers. If the medium is free for a specified time, (called Distributed Inter Frame Space (DIFS), in the standard), then the station is allowed to transmit, the receiving station will perform a cyclic redundancy check (CRC) of the received packet and send an acknowledgement packet (ACK). Receipt of the acknowledgment will indicate to the transmitter that no collision occurred. If the sender does not receive the acknowledgment, then it will retransmit the fragment until it gets acknowledged or thrown away after a given number of retransmissions.
In order to reduce the probability of two stations colliding because they cannot hear each other, the standard defines a Virtual Carrier Sense mechanism. A station willing to transmit a packet will first transmit a short control packet called Request To Send (RTS), which will include the source, destination, and the duration of the following transaction, (i.e., the packet and the respective ACK), the destination station will respond (if the medium is free) with a response control Packet called Clear to Send (CTS), which will include the same duration information.
All stations receiving either the RTS and/or the CTS, will set their Virtual Carrier Sense indicator, (called Network Allocation Vector (NAV)), for the given duration, and will use this information together with the Physical Carrier Sense when sensing the medium. This mechanism reduces the probability of a collision on the receiver area by a station that is “hidden” from the transmitter, to the short duration of the RTS transmission, because the station will hear the CTS and “reserve” the medium as busy until the end of the transaction. The duration information on the RTS also protects the transmitter area from collisions during the ACK, (by stations that are out of range from the acknowledging station).
It should also be noted that because of the fact that the RTS and CTS are short frames, it also reduces the overhead of collisions, since these are recognized faster than it would be recognized if the whole packet was to be transmitted, (this is true if the packet is significantly bigger than the RTS, so the standard allows for short packets to be transmitted without the RTS/CTS transaction, and this is controlled per station by a parameter called RTS Threshold).
The Standard defines four (4) different types of Inter Frame Spaces, which are used to provide different priorities.
A Short Inter Frame Space (SIFS) is used to separate transmissions belonging to a single dialog, (e.g., Fragment-ACK), and is the minimum Inter Frame Space. There is always at most one single station to transmit at this given time, hence having priority over all other stations.
A Point Coordination IFS (PIFS) is used by the AP (or Point Coordinator, as called in this case), to gain access to the medium before any other station.
A Distributed IFS (DIFS) is the Inter Frame Space used for a station willing to start a new transmission, which is calculated as PIFS plus one slot time, i.e. 128 microseconds.
An Extended IFS (EIFS) is a longer IFS used by a station that has received a packet that could not be understood. This is needed to prevent the station (who could not understand the duration information for the Virtual Carrier Sense) from colliding with a future packet belonging to the current dialog.
Backoff is a well known method to resolve contention between different stations willing to access the medium, the method requires each station to choose a Random Number (n) between 0 and a given number, and wait for this number of slots before accessing the medium, always checking whether a different station has accessed the medium before.
The slot time is defined in such a way that a station will always be capable of determining if another station has accessed the medium at the beginning of the previous slot. This reduces the collision probability by half.
Exponential backoff occurs each time the station chooses a slot and happens to collide whereby the station will increase the maximum number for the random selection exponentially. An exponential backoff algorithm must be executed when the station senses the medium before the first transmission of a packet, and the medium is busy, after each retransmission, and after a successful transmission. The only case when this mechanism is not used is when the station decides to transmit a new packet and the medium has been free from more than DIFS.
EDCA introduces the concept of traffic categories. Each WTRU has four traffic categories, or priority levels. Using EDCA, the WTRUs try to send data after detecting that the medium is idle and after waiting a period of time defined by the corresponding traffic category, called the AIFS. A higher-priority traffic category has a shorter AIFS than a lower-priority traffic category. Thus, WTRUs with lower-priority traffic must wait longer than those with high-priority traffic before trying to access the medium. This is fixed per access category and is a very short duration of time.
To avoid collisions within a traffic category, the WTRU counts down an additional random number of time slots, known as a contention window, before attempting to transmit data. This can also be defined per access category. If another WTRU transmits before the countdown has ended, the WTRU waits for the next idle period, after which it continues the countdown where it left off. No guarantees of service are provided, but EDCA establishes a probabilistic priority mechanism to allocate bandwidth based on traffic categories.
In a WLAN that is compliant with the IEEE 802.11e specification, different types of traffic are mapped into corresponding access categories with corresponding priorities. Each access category has a different minimum contention window size and a maximum contention window size which reflect the priority of that category, as compared to an 802.1a/b/g WLAN network. The contention window size refers to the delay between packet transmissions. As the contention window size changes, so does the AIFS in a proportional manner.
As different traffic users contend for access to a channel, the different minimum contention window size provide a clear advantage for higher priority access categories over the lower priority access categories. However, the WLAN is not prevented from reaching a congested state, and the WLAN does not have a mechanism to control congestion once it arises.
Since an increase in the number of users associated with any access category results in an increase in the number of collisions and a corresponding increase in packet error rate (PER), the system would inevitably be driven into a congested state.